Electroabsorption modulators (EAMs) have been shown to be useful in fiber-optic links operating near either 1.3 or 1.55 μm for both analog and digital signal transmission. They are small in size and can be integrated with a laser diode, for example, while being very effective in changing the light intensity as a function of the applied electric field. In particular, EAMs that use multiple quantum well active regions are currently very popular. As those skilled in the art will understand, a quantum well is a material unit that has a very thin “well” material (typically of the order of 100 Angstrom) surrounded by potential energy barriers. Inside the quantum well, particle, such as electrons or holes, are restricted in motion in the dimension perpendicular to the well and has well-defined quantized states with respect to momentum and energy. The particles are free to move around in the other two directions. When these quantum wells are stacked together without interactions between each other, this resultant structure is referred to as multiple quantum wells (MQW). The MQW provides a large absorption-coefficient change via the quantum-confined Stark effect (QCSE), which is the shift of the excitonic (the resonance state of electron-hole pairs) absorption peak of the quantum well under the influence of an applied electric field. Typically, the peak shifts to lower energy when an electric field is applied.
An analog fiber link is an optical fiber communication channel for transmitting analog signals. For an externally modulated analog fiber link, which uses a transmitter with the light modulation occurring outside the laser source, increasing the received optical power reduces the link loss, which is the ratio of the input signal intensity to the output signal intensity. The output signal of the receiver is proportional to the square of the optical power. The optical power used in the link, however, is currently limited by the optical saturation properties of the EAM. An optical saturation occurs when the output signal intensity is no longer linearly proportional (or becomes sub-linear) to the input signal power to the EAM. Consequently, a concern for the MQW EAMs is their relatively low saturation optical power, where optical saturation begins. This power level can be determined from the optical power when the output signal intensity is 1 dB below that which corresponds to unsaturated modulation. The conventional MQW EAMs, particularly those made of InGaAs/InP, tend to saturate at a much lower level.
In quantum wells (QWs), the barriers hinder the sweep-out of the photo generated electrons and holes, particularly holes, resulting in carrier pile-up near the barriers. The traditional approach to reduce this effect had been to use InGaAsP or InAlAs (or InGaAlAs) instead of InP as barrier materials to reduce the valence band offset, which was shown to improve the optical saturation of the MQW EAMs. Also, there have been attempts to use strain-compensated InGaAsP/InGaAsP and InAsP/GaInP quantum wells, which have shallow wells, to improve the saturation optical power at 1.55 and 1.3 μm. A strained quantum well refers to a quantum well where either the barrier or the quantum well is lattice mismatched to the substrate so that the quantum well as a unit is under (tensile or compressive) hydrostatic strain. A strain compensated quantum well refers to a quantum well structure in which the well and the barrier are oppositely (compressive versus tensile) strained so that the net strain in one unit of quantum well is zero. Although it has been reported that the MQW EAMs with InGaAs/InAlAs can handle optical power up to 40 mW without degradation in the bandwidth, the link gain at RF frequencies was observed to saturate at a much lower level. The maximum optical power that does not cause RF gain saturation is currently limited to approximately 10 mW.
It has also been observed that increasing the electric field reduces the screening effect due to spatially distributed holes that cannot be drifted out of the quantum well (their presence causes an effective reduction of the applied electric field). Hence in order to increase the saturation optical power further, the operating bias must be increased for a given intrinsic layer thickness without compromising the modulator performance such as modulator slope efficiency, which is defined as the maximum change in optical transmission versus change in applied voltage.